Known sensors for measurement of coolant/fluid levels in a nuclear reactor module typically are invasive sensors, for example, differential pressure gauges, that must reside inside pressurized modules and must penetrate the module walls, for example, of concentric cylindrical vessels or a single cylindrical vessel, each having, for example, an outer vertical wall comprising, for example, an outer containment pressure vessel (CPV), sometimes referred to as simply a containment vessel or (CNV) and an inner reactor pressure vessel (RPV). Typical pressure ranges in a reactor pressure vessel are on the order of 1800 to 2250 PSI and may be within a larger range of 1000 to 3000 PSI. For example, reactor steam pressure may be typically measured by one or more pressure gauges of an RPV.
It is generally necessary that a nuclear reactor system be capable of providing measurements of the fluid level inside a reactor pressure vessel to an accuracy of a few centimeters, for example, for normal regulation of fluid level within a nuclear reactor module. An invasive fluid measurement system is typically used so that fluid level may be controlled, for example, via a Chemical Volume and Control System (CVCS), alternatively referred to as a Makeup/Letdown or a Makeup and Purification System. In the present patent application and claims, Chemical Volume and Control System shall be used generically to refer to all such systems. The CVCS circulates continuously to control both water chemistry and coolant fluid level. There are typically separate inlet and outlet lines for the CVCS. The CVCS typically utilizes fluid flow rate sensors to measure inlet and outlet flow rates. One purpose of the CVCS is to provide a means of regulating fluid level in a reactor so that the fluid level remains within a normal operating band. It is normal for a CVCS to add fluid or remove fluid, for example, from a reactor pressure vessel in order to maintain fluid level in a vessel within the operating band. On the other hand, a leak from a vessel, such as a reactor pressure vessel (RPV), to an external vessel, such as a containment vessel may be indicated by a drop in coolant level below the operating band lower limit. In such a situation, a lowering of the center of mass of the fluid contained within the reactor may indicate a leak, rather than normal fluid level regulation. The loss of fluid sometimes referred to as a LOCA (loss of coolant accident) may require immediate intervention, for example, initiation of an emergency core cooling system, remedial action and possible reactor shut-down and repair.
In a NuScale small modular reactor (SMR), for example, the CVCS lines penetrate both the RPV and the CPV, but the lines are not used to measure coolant level; (invasive level measurement systems introduced above may be used).
Two types of known invasive sensors used are differential pressure gauges for measuring pressure levels in a pressurized module and heated thermocouples. Another type utilizes a guided wave radar device that penetrates the top of the reactor module to measure the water level in the reactor module. Typically, for installation of either known invasive sensor type (or the radar type), the pressurized modules are pre-drilled to allow the invasive sensors to operate within the reactor module, which is typically cylindrical, and report the measurement result to systems outside the reactor module. The pre-drilling may increase the risk of the undesirable result of a leakage through the reactor module wall(s). A leak may take a long time to develop and so measurements are often taken as a time series. A slow loss of fluid may simply be a sign of normal fluid regulation by the CVCS. So factors in distinguishing normal operation from abnormal operation may include, but not be limited to, the rate of loss of fluid from the reactor module or the fluid level reaching a low fluid level below the band lower limit or the fluid level reaching a fluid level that does not compare with a computer processor estimated fluid level considering other systems such as flow rate, temperature and pressure systems other than the CVCS. The pressure within the pressure vessel may precipitate a vessel failure caused by the invasive nature of any invasive sensor installation through a wall of the reactor module.
One may recall the partial meltdown that occurred at the Three Mile Island nuclear reactor in Pennsylvania in 1979. It has been reported that the accident at Three Mile Island was caused by operator error based upon an erroneous estimate of coolant fluid levels in the reactor core. A coolant fluid level regulation problem may be a serious problem that requires a more efficient and accurate solution than an invasive sensor solution to the problem of monitoring fluid level in a reactor system.
Consequently, there is a need for an alternative method and apparatus and system for sensing fluid levels, for example in nuclear reactors, that is more accurate and more efficient than known invasive methods and systems for measuring fluid levels in a tank, such as are used in nuclear reactors.